Fluid working machines are used in various technical fields. For example, they are used if the pressure of a fluid has to be increased. In this case, the fluid working machine is generally referred to as a pump. Also, in some fields fluid working machines are used to generate mechanical power out of pressurized fluid. Here, the pressure of the fluid is converted into mechanical energy. This type of fluid working machine is generally referred to as a fluid motor. More advanced fluid working machines can even combine both functionalities. Depending on the mode in which they are used, they either work as a fluid motor or as a fluid pump.
Depending on the technical field, the fluid can be a liquid (for example hydraulic oil or even water in the field of hydraulics) or a gas. Of course, mixtures between fluid and gas and hypercritical fluids (where a distinction between gas and liquid cannot be made anymore) can be used as well. It should be noted that it is also possible that a certain amount of solid-state particles can be contained in the fluid as well.
One particular subset of fluid working machines are the so-called synthetically commutated fluid working machines that are also known as DDP (DDP for Digital Displacement® pump). These synthetically commutated hydraulic machines form a unique type of fluid working machines. Instead of passive valves that open or close under the influence of a differential pressure on both sides of the respective valve, at least a part of the valves are replaced by actuated valves. Typically, if the fluid working machine is solely used as a hydraulic pump, only the low-pressure valves are replaced by actuated valves. The basic principle is that the inlet valve that is fluidly connected to a working chamber can remain open after the working chamber has reached its maximum volume and starts to contract again. If the fluid valve remains open for the complete contracting cycle, the fluid is simply pushed back into the low-pressure fluid reservoir, without performing any useful work (and without consuming a significant amount of mechanical work). If, on the other hand, the fluid valve is closed at the very beginning of the contracting cycle, the fluid in the working chamber is pressurized and ejected through a high-pressure valve (that can be a passive valve in the case of a fluid pump). Another option is to close the fluid valve somewhere between these two extremes. In this case, an essentially arbitrary partial volume of the working chamber can be pressurized and pumped to the high-pressure side of the fluid pump.
If the high-pressure valves (i.e. those fluid valves that are connected to the high-pressure fluid manifold) are replaced by actuated valves as well, a fluid motor or a combined fluid pump and motor can be realised that is able of part-stroke pumping and/or part-stroke motoring modes.
Such synthetically commutated fluid working machines are known in the state of the art for several years.
A particular problem of the synthetically commutated fluid working machine design is that actuated valves are needed through which significant fluid flow rates can be passed through in two opposite directions. Of course, a fluid flow in a certain direction should not lead to an unwanted switching of the respective valve due to forces induced by the fluid flowing through the respective actuated valve (which usually relates to its open state). This is in particular true for the ejection fluid flow through a low-pressure valve during the exhaust cycle of an idle-stroke (no effective pumping done) or a part-stroke cycle (where only a part of the working chamber's fluid volume is pumped to the high-pressure side), when the valve is still open and not yet energised to be closed.
In such a case, significant fluid flow forces can act on the valve that can lead to an early/unwanted closure of the respective valve, in particular if the fluid working machine is operated at higher rotational speed (rpm, for example).
The same can be true for the high-pressure side, if actuated valves are used for the high-pressure side as well (which is usually the case for fluid motors or combined fluid motors and pumps).
A way to address this problem was suggested in the international patent application WO 2010/073040 A1. Here, it was suggested (among other possibilities) to use a fluid barrier upstream of the valve's poppet to “shield” the valve poppet from fluid forces induced by a fluid flow, flowing in the closing direction of the valve poppet.
Experience has shown that these fluid barriers (sometimes addressed as “fluid hats”) work well in practice. However, it is still problematic to manufacture actuated valves, comprising such fluid hats in an effective, reliable, cost-effective and serviceable way. For manufacturing reasons, it is essentially unavoidable to provide the fluid hats as a separate part that has to be attached somehow to the valve body during the manufacturing process. Of course, the method for connecting the fluid hat to the valve's body should be as simple and as fast as possible. On the other hand, one has to realise that a significant force is exerted frequently on the fluid hat during operation of the fluid working machine. This frequent mechanical load should not result in a loosening of the fluid hat (or parts of it), since this can compromise the operation of the whole fluid working machine and can even lead to a destruction of the complete fluid working machine. In reality, it is therefore advisable to exchange the fluid hats after a certain operational time span. This leads to the auxiliary condition that a servicing of the fluid working machine (in particular an exchange of the fluid hat) should be possible without too many efforts.
Fulfilling all these conditions at the same time is a remarkable task where so far no convincing solution has been proposed.
For example, it was proposed to attach the fluid hat through anchoring webs, where the ends of the anchoring webs are spaced between two parts of the fluid working machine that are attached to each other by means of a thread. However, replacement of a fluid hat that is attached in this way requires a removal of significant parts of the fluid working machine. This results in significantly increased service cost.